Aluminized 19AP4?
For what it is worth, my old GE tube manual shows the following for the 19AP4:
19AP4; clear glass, single magnet ion trap.
19AP4A; gray glass, single magnet ion trap.
19AP4B; gray frosted glass, single magnet ion trap.
19AP4C; gray glass, aluminized, single magnet ion trap.
19AP4D; clear frosted glass, single magnet ion trap.
I bet that some were also rebuilt with straight guns, as well.
Poor/complicated layout?
To me, it really looks like a "factory kludge"... The chassis still has the punch outs for the (5) 25Z6 rectifier tubes used in the 10 inch models plus other unused punch outs.
jr